Covert Critters

We critters who live on the surface of planet Earth can become quite provincial about our kind—by being quite unaware of the many life forms that live underground. For example, people see other people as the most important critters on the planet. That's probably true for other surface species. We see each other, interact with each other, and conclude that's about all there is to it.

But there's a vast biosphere that is hidden from us, just below our feet. It might as well be covert. All of us surface critters live in an extremely thin layer on the surface of the planet. The atmosphere and the exposed land are like the thin skin of an apple—there's so much more below the skin. For example, subsurface fresh water—existing down to many miles—has a volume of about 100 times that of surface fresh water. Subsurface life has a mass of some 20 times all that lives in the ocean and makes up about one-third of all the planet's biomass. That's pretty impressive!

Never experiencing the sun, subsurface life cannot depend on photosynthesis, as must all surface life. Photosynthesis in plants enables them to manufacture sugars for themselves, as well as all other critters of the light, who eat plants or other plant-eating creatures. How do the covert critters survive without light and its resultant photosynthetic sugars? They feed on dissolved minerals in the water, as well as raw hydrogen. Thus they are reliant on biochemical reactions, which are much less efficient than photosynthesis. As a result, subsurface life exists in the slow lane. Underground microbes (and most life down there is in the form of microbes) may divide once every thousand years or so. Contrast that to surface E.coli, which divide every 20 minutes! This is a pace of life some 30 million times faster!

Moreover, underground life-forms are much tougher than their surface cousins. In 2003 the space shuttle Columbia broke up during reentry into the atmosphere. Aboard Columbia was a biological experiment on nematodes—very hardy, underground tiny worms. They survived the fiery explosion and they survived the 25 mile plunge to the ground where, a few weeks later—when they were subsequently located—they were reproducing!

Earth's subsurface life-forms are closer in character and metabolism to the earliest kinds of life on the planet, thus recent discoveries of these covert critters are helping scientists to understand how those deep ancestral creatures survived and thrived. Our expanding comprehension of the limits of metabolism of life on Earth is preparing us to better understand what kinds of life (if any) we'll find on Mars, or Jupiter's moon Europa, or Saturn's moon Enceladus—most all of which is likely to be subsurface, if it exists.

These findings make us wonder about what other kinds of critters we'll yet find on (or under) Earth. With each passing year we expand the limits of what we know to be life.